1. Field of the Invention
The present invention is concerned with the field of induction plasma torches and relates more specifically to a plasma torch of which the performance is improved by using a plasma confinement tube made of ceramic material and cooled through a high velocity fluid flowing into a thin annular chamber enveloping the outer surface of that tube.
2. Brief Description of the Prior Art
Induction plasma torches have been known since the early sixties. Their basic design has however been substantially improved over the past thirty years. Examples of prior plasma torch designs are described in British patent No. 1,061,956 (Cleaver) published on Mar. 15, 1967, in U.S. Pat. No. 3,694,618 (Poole et al.) dated Sep. 26, 1972, and in U.S. Pat. No. 3,763,392 (Hollister) of Oct. 2, 1973. The basic concept of an induction plasma torch involves an induction coupling of the energy into the plasma using a 4-6 turns induction coil. A gas distributor head is used to create a proper flow pattern into the region of the produced plasma, which is necessary to stabilize the plasma confined in a tube usually made of quartz, to maintain the plasma in the center of the coil and protect the plasma confinement tube against damage due to the high heat load from the plasma. At relatively high power levels (above 5-10 kW), additional cooling is required to protect the plasma confinement tube. This is usually achieved through dionized water flowing on the outer surface of the tube.
Numerous attempts have been made to improve the protection of the plasma confinement tube. These tentatives are concerned with the use of (a) a protective segmented metallic wall insert inside the plasma confinement tube [U.S. Pat. No. 4,431,901 (Hull) issued on Feb. 14, 1984], (b) porous ceramic to constrict the plasma confinement tube [J. Mostaghimi, M. Dostie, and J. Jurewicz, "Analysis of an RF induction plasma torch with a permeable ceramic wall", Can. J. Chem. Eng., 67, 929-936 (1989)], and (c) radiatively cooled ceramic plasma confinement tubes [P. S. C. Van der Plas and L. de Galan, "A radiatively cooled torch for ICP-AES using 1 liter per min of argon", Spectrochemica Acta, 39B, 1161-1169 (1984) and P. S. C. Van der Plas and L. de Galan, "An evaluation of ceramic materials for use in non-cooled low flow ICP torches", Spectrochemica Acta, 42B, 1205-1216 (1987)]. These attempts each present their respective limitations and shortcomings.
The use of a segmented metallic wall insert to improve protection of the plasma confinement tube present the drawback of substantially reducing the overall energy efficiency of the plasma torch.
It has been found that a plasma confinement tube made of porous ceramic material offers only limited protection.
Concerning the radiatively cooled confinement tubes, their ceramic materials must withstand the relatively high operating temperatures, exhibit an excellent thermal shock resistance and must not absorb the RF (radio frequency) field. Most ceramic materials fail to meet with one or more of these stringent requirements.